This invention relates to an engine-driven refrigeration unit, and in particular to a refrigeration unit having a first heat exchanger functioning as either a condenser or an evaporator. The first heat exchanger receives water for either condensing or evaporating refrigerant supplied thereto.
Engine-driven refrigeration units, such as those employed on board boats or ships, generally require some means to defrost the evaporator of the unit. Sometimes the unit is operated in a reverse or heat pump mode to defrost the evaporator. In the defrost mode, a heat exchanger, normally functioning as the refrigeration unit evaporator, functions as a refrigerant condenser, with the first heat exchanger, normally functioning as the condenser, thence operating as an evaporator. During the defrost mode, heat is absorbed from the nominal "condensing medium" and transferred to the refrigerant, with the vaporized refrigerant thereafter rejecting heat to defrost the coils of the "condenser". On ship board units, water from any source, such as an ocean, lake or river, (hreinafter collectively referred to as "sea water") is preferably employed as the condensing or evaporating medium for the first heat exchanger. The temperature of the sea water may vary through a relatively broad range depending upon ambient temperature. As an example, in relatively warm climates, sea water temperature may exceed 31.degree. C., whereas in relatively cold climates the temperature of the water may fall to 4.degree. C. or even lower. When the sea water is employed as a source of heat during the defrost mode of operation of the refrigeration unit, the temperature of the water is substantially reduced as it flows through the heat exchanger functioning as the evaporator. If the initial temperature of the water is 15.degree. C. or lower, the water may not contain sufficient heat to permit efficient and effective defrosting of the coils. In effect, during the defrost operation, heat is transferred from the water to the refrigerant. Heat is rejected by the refrigerant in the second heat exchanger. With a relatively small amount of heat available in the sea water, the defrosting operation will take a relatively long period of time. An increase in the temperature of the refrigerated cargo may occur if the defrosting operation is unduly prolonged. With highly perishable goods or goods requiring rigid temperature control, any temperature increase resulting from an excessively long defrost operation is undesirable, and in some applications intolerable.